CONDENSED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Common Stock	Additional Paid-in Capital	Accumulated Deficit	Accumulated Other Comprehensive Income	Total
Beginning balance at Dec. 31, 2017	$ 24	$ 712,165	$ (604,494)		$ 107,695
Beginning balance (in shares) at Dec. 31, 2017	24,453
Issuance of common stock under employee stock purchase plan		81			81
Issuance of common stock under employee stock purchase plan (in shares)	7
Issuance of common stock upon exercise of common stock options and warrants	$ 3	9,588			9,591
Issuance of common stock upon exercise of common stock options and warrants (in shares)	2,551
Issuance of common stock for services rendered		23			23
Issuance of common stock for services rendered (in shares)	1
Stock-based compensation		1,589			1,589
Net loss			(20,095)		(20,095)
Ending balance at Mar. 31, 2018	$ 27	723,446	(624,589)		98,884
Ending balance (in shares) at Mar. 31, 2018	27,012
Beginning balance at Dec. 31, 2017	$ 24	712,165	(604,494)		107,695
Beginning balance (in shares) at Dec. 31, 2017	24,453
Net loss					(47,731)
Ending balance at Sep. 30, 2018	$ 27	727,064	(652,225)		74,866
Ending balance (in shares) at Sep. 30, 2018	27,179
Beginning balance at Mar. 31, 2018	$ 27	723,446	(624,589)		98,884
Beginning balance (in shares) at Mar. 31, 2018	27,012
Issuance of common stock under employee stock purchase plan		78			78
Issuance of common stock under employee stock purchase plan (in shares)	6
Issuance of common stock upon exercise of common stock options and warrants		575			575
Issuance of common stock upon exercise of common stock options and warrants (in shares)	151
Issuance of common stock for services rendered		22			22
Issuance of common stock for services rendered (in shares)	2
Stock-based compensation		1,538			1,538
Net loss			(16,031)		(16,031)
Ending balance at Jun. 30, 2018	$ 27	725,659	(640,620)		85,066
Ending balance (in shares) at Jun. 30, 2018	27,171
Issuance of common stock under employee stock purchase plan		46			46
Issuance of common stock under employee stock purchase plan (in shares)	5
Issuance of common stock for services rendered		32			32
Issuance of common stock for services rendered (in shares)	3
Stock-based compensation		1,327			1,327
Net loss			(11,605)		(11,605)
Ending balance at Sep. 30, 2018	$ 27	727,064	(652,225)		74,866
Ending balance (in shares) at Sep. 30, 2018	27,179
Beginning balance at Dec. 31, 2018	$ 27	728,342	(664,375)		$ 63,994
Beginning balance (in shares) at Dec. 31, 2018	27,188,000				27,188,000
Sale of common stock, net of issuance costs	$ 1	1,584			$ 1,585
Sale of common stock, net of issuance costs (in shares)	533,000
Issuance of commitment shares (in shares)	270,000
Issuance of common stock under employee stock purchase plan		26			26
Issuance of common stock under employee stock purchase plan (in shares)	11,000
Issuance of common stock for services rendered		23			23
Issuance of common stock for services rendered (in shares)	6,000
Stock-based compensation		1,016			1,016
Unrealized gain on marketable securities				$ 2	2
Net loss			(10,974)		(10,974)
Ending balance at Mar. 31, 2019	$ 28	730,991	(675,349)	2	55,672
Ending balance (in shares) at Mar. 31, 2019	28,008,000
Beginning balance at Dec. 31, 2018	$ 27	728,342	(664,375)		$ 63,994
Beginning balance (in shares) at Dec. 31, 2018	27,188,000				27,188,000
Sale of common stock, net of issuance costs (in shares)					785,848
Net loss					$ (33,283)
Ending balance at Sep. 30, 2019	$ 29	735,254	(697,658)	1	$ 37,626
Ending balance (in shares) at Sep. 30, 2019	28,858,000				28,858,000
Beginning balance at Mar. 31, 2019	$ 28	730,991	(675,349)	2	$ 55,672
Beginning balance (in shares) at Mar. 31, 2019	28,008,000
Sale of common stock, net of issuance costs	$ 1	2,271			2,272
Sale of common stock, net of issuance costs (in shares)	786,000
Issuance of common stock under employee stock purchase plan		42			42
Issuance of common stock under employee stock purchase plan (in shares)	19,000
Issuance of common stock for services rendered		36			36
Issuance of common stock for services rendered (in shares)	14,000
Stock-based compensation		889			889
Net loss			(11,176)		(11,176)
Ending balance at Jun. 30, 2019	$ 29	734,229	(686,525)	2	47,735
Ending balance (in shares) at Jun. 30, 2019	28,827,000
Issuance of common stock under employee stock purchase plan		29			29
Issuance of common stock under employee stock purchase plan (in shares)	15,000
Issuance of common stock upon exercise of warrants (in shares)	4,000
Issuance of common stock for services rendered		33			33
Issuance of common stock for services rendered (in shares)	12,000
Stock-based compensation		963			963
Unrealized gain on marketable securities				(1)	(1)
Net loss			(11,133)		(11,133)
Ending balance at Sep. 30, 2019	$ 29	$ 735,254	$ (697,658)	$ 1	$ 37,626
Ending balance (in shares) at Sep. 30, 2019	28,858,000				28,858,000